Lighting units or luminaires with optically active layers are known in the art. U.S. Pat. No. 7,631,980, for instance, targets at a luminaire comprising a light source and a lighting panel in front of the light source, wherein the outer side (front side) of the lighting panel has a profiled surface in order to direct the transmitted light radiation mainly into a predetermined zone, wherein the light radiation is at relatively small angles to the direction perpendicular to the plane of the lighting panel, and wherein light radiation at small angles to the plane of the lighting panel is reduced, in particular when the luminaire has to produce a relatively high intensity of light radiation in the predetermined zone. To achieve this target, walls of a less translucent material are embedded in the transparent material of the lighting panel, which walls extend substantially perpendicularly to the plane of the lighting panel and preferably extend over the entire thickness of the lighting panel, i.e. over the thickness of the transparent material of the lighting panel. According to U.S. Pat. No. 7,631,980, such walls were found to be an effective means for further reducing inconvenient light radiation from the lighting panel.
Further according to U.S. Pat. No. 7,631,980, it was found that light radiation that leaves the profiled surface at the front of the lighting panel at a relatively small angle to the plane of the lighting panel, which light radiation causes inconvenient glare, and has followed long paths through the material of the lighting panel. Such long paths are possible for certain light rays in the transparent material of the lighting panel, in particular light rays directed at small angles to the plane of the lighting panel. According to U.S. Pat. No. 7,631,980, such light rays appear to be an undesired side effect of the structure of the profiled surface. Such undesired light rays may also be caused by reflection or scattering of light radiation by dust and other particles on the surface of the lighting panel, or by damage or irregularities in the material of the lighting panel.